Paid Memberships Pro customization to only let members of a certain level checkout if a discount code was used.

my-pmpro-levels-require-dc.php

<? // Updated for Multiple Levels.
/*
  Require a discount code for specific membership levels.
  Place this code in your active theme's functions.php or a custom plugin.
*/

function my_pmpro_registration_checks_require_code_to_register($pmpro_continue_registration)
{
    // Only proceed if registration checks have passed so far
    if (!$pmpro_continue_registration) {
        return $pmpro_continue_registration;
    }

    // Define the levels that require a discount code
    $levels_require_discount = array(1, 2, 3); // Replace with the IDs of the levels that require a discount code

    // Access the global variables for level and discount code
    global $pmpro_level, $discount_code;

    // Check if the current level requires a discount code and if the discount code is empty
    if (in_array($pmpro_level->id, $levels_require_discount) && empty($discount_code)) {
        pmpro_setMessage("You must use a valid discount code to register for this level.", "pmpro_error");
        return false;
    }

    return $pmpro_continue_registration;
}
add_filter("pmpro_registration_checks", "my_pmpro_registration_checks_require_code_to_register");